That is because scientists say mercury from coal accumulates in many fish. Children and babies exposed to the metal, through mother's milk or eating contaminated fish, are at risk of learning and developmental problems. Adults who eat too many of the fish also face risks....

When combined with the EPA's other current and coming rules on "criteria" pollutants, like ones that cause acid rain and smog, the mercury measure would force utilities to invest tens of millions of dollars on technologies to remove the substances....

Francois Broquin, a co-author of reports on coal by Bernstein Research, said the combined rules could push as much as 20 percent of U.S. coal-fired electric generation capacity to retire by 2015. "Obviously that will have an impact," he said.

Frank O'Donnell, the president of the environmental group Clean Air Watch, said that if a large chunk of the coal-fired power fleet went into retirement it could help the country exceed Obama's goal of reducing greenhouse gas emissions by about 17 percent from 2005 levels by 2020....

Utilities would likely build plants to burn natural gas, which emits half the carbon that coal does, as the main alternative. Alternative energy like wind and solar power, which provided the most new U.S. electricity capacity last year, could also become more attractive to utilities.

A reduction of 17% (or somewhat less than that) by 2020 is well below what is needed to prevent the worst effects of climate change. However, it is better than nothing and could build some momentum for better climate change policy. Perhaps in future years the political climate will be more favorable to passing a climate change law with deeper cuts.